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 Memory Management  |  Question 57 of 114    Print  
How does Space allocation table place within a block ?
 
Each block contains entries as follows
Fixed block header
Variable block header
Row Header,row date (multiple rows may exists)
PCTEREE (% of  free space for row updation in future)



  
Total Answers and Comments: 1 Last Update: October 03, 2006   
  
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October 03, 2006 16:25:38   #1  
Anu        

RE: How does Space allocation table place w...

A block is divided into

1. Header

2.free space

3.data space

The free space is in the middle shrinks as header and data space grow.

Space usage in the data space :

The data space is managed with 2 parameters

a.pctused    b.pctfree

suppose pctused=40 and pctfree=20

 We know data is stored in the data block in database.The data or rows are filled in the pctused area and they get filled until 80% of the dataspace is full ie100-pctfree.ie Row inserts occur until 80% of dataspace is full.

     The use of pctfree is to accomadate any growth in dataspace that might occur due to row updation or any previous null values replaced with values.

       A pctused can also shrink due to any deletion.Suppose a deletion causes pctused to a value 60% from 80%.Now this block is not available for new inserts until pctused is 40%.So when pctused=40 then the block is included into freelist and made available for new inserts.

100-pctfree.


 
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